Improve BlobController show.json performance: cache method calls

What does this MR do?

A (possible) set of "low hanging fruit improvements".

I mostly tested it on gitlabhq/-/blob/master/CHANGELOG.md locally, as I found it to be "appropriately" slow to work from there. It takes ~40 seconds locally.

1. Avoid Project.markdown_reference_pattern repeated call?

Looking into the flame graph, it would allow us to save ~3 seconds, as it is called in the cycle:

After caching the value, we see that Banzai::Filter::ProjectReferenceFilter#call is faster now:

2. The same is for User.reference_pattern

(check the screenshot above)

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Maybe we should deal with class methods that only generate strings through all the codebase?
Seems that we have quite a few of those, especially regexes.
It could be caching or moving them into const.
We could also create a rubocop's cop which checks that.

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry
• [-] Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• [-] Database guides
• [-] Separation of EE specific content

Availability and Testing

N/A

Security

N/A

Related to #217580 (closed)